A. Technical Field
The present invention relates to: a production process for a water-absorbing agent which is favorably used for sanitary materials such as paper-made diapers (disposable diapers), sanitary napkins and so-called incontinent pads; and a water-absorbent resin (water-absorbing material) composition.
B. Background Art
In recent years, water-absorbent resins that are hydrophilic resins are widely used as constituent materials of sanitary materials, such as disposable diapers, sanitary napkins, and so-called incontinent pads, for the purpose of causing the water-absorbent resins to absorb body fluids.
Known examples of the above water-absorbent resins are as follows: crosslinked products of partially neutralized polyacrylic acids; hydrolyzed products of starch-acrylonitrile grafted polymers; neutralized products of starch-acrylic acid grafted polymers; saponified products of vinyl acetate-acrylic acid ester copolymers; hydrolyzed products of acrylonitrile- or acrylamide copolymers, and their crosslinked products; and crosslinked polymers of cationic monomers.
Examples of the properties which the above water-absorbent resins should have are as follows: upon contact with aqueous liquids such as body fluids, excellent water absorption amount or speed, the liquid permeability, the gel strength of the swollen gel, the suction power to suck up water from a base material containing aqueous liquids. However, relations between these properties do not necessarily display positive correlations. For example, as the absorption capacity under no load increases, the absorption properties under a load deteriorate.
As to a method for improving such water absorption properties (e.g. absorption capacities under no load and under a load) of the water-absorbent resin in good balance, there is a known art in which the neighborhood of the surface of the water-absorbent resin is crosslinked, and various methods have been proposed as such.
For example, there are known methods in which the following materials are used as the crosslinking agents: polyhydric alcohols (JP-A-58-180233 and JP-A-61-016903); polyglycidyl compounds, polyaziridine compounds, polyamine compounds, or polyisocyanate compounds (JP-A-59-189103); glyoxal (JP-A-52-117393); polyvalent metals (JP-A-51-136588, JP-A-61-257235 and JP-A-62-007745); silane coupling agents (JP-A-61-211305, JP-A-61-252212, and JP-A-61-264006); alkylene carbonates (DE 4020780). In addition, there are also known methods in which the following materials are allowed to be present as third substances for the purpose of improving the dispersibility of the crosslinking agent when the crosslinking agent is mixed or when the crosslinking reaction is carried out: inert inorganic powders (JP-A-60-163956 and JP-A-60-255814); dihydric alcohols (JP-A-01-292004); water along with ether compounds (JP-A-02-153903); alkylene oxide adducts of monohydric alcohols, organic acid salts, lactams, and so on (EP 555692); and phosphoric acid (Publication of Internal Patent Application as entered the national phase in Japan (Kohyo) No. 08-508517).
The production of various water-absorbent articles containing water-absorbent resins needs a step of combining a high hygroscopic resin with a fibrous material. Cases of the production of recently trendy water-absorbent articles, which use a large quantity of water-absorbent resin and are getting thinner and thinner, such as sanitary supplies, have more and more serious problems in that the efficient and stable production is impossible according to working environment and weather conditions, because the amount of the resin as added per piece of product is increasing. That is to say, resins of which the particles tend to block each other, therefore, of which the so-called blocking ratio under a load is high, have more and more serious problems in that: when a pressure is applied to particles of the resins under conditions of specific humidity, the resins easily cause blocking in hoppers or on the way of lines, therefore stable operation is difficult.
Such a blocking ratio under a load might have a tendency to worsen due to crosslinking the neighborhood of the surface of the above water-absorbent resin, and further has a tendency to be contradictory to the water absorption properties under a load.
Generally, examples of known methods of post-treatment to solve the blocking property of resins include: a composition as obtained by mixing a water-absorbent resin powder and a hydrophobic, finely particulate silica in a specific ratio (JP-B-61-017542); a composition as obtained by mixing a water-absorbent resin powder with an inorganic powder such as hydrous silicon dioxide, hydrous aluminum dioxide, and hydrous titanium dioxide (JP-A-59-080459); a method comprising the steps of treating a water-absorbent resin with a specific cationic surfactant and then mixing the treated resin with an inorganic substance or a high melting point organic compound (JP-A-61-069854); a method comprising the step of mixing a water-absorbent resin powder with stearic acid and an inorganic powder (JP-A-63-105064); and a method comprising the step of treating a water-absorbent resin with a specific silicone surfactant (JP-A-07-165981).
However, as to such resins with the improved blocking property, it has been found that the balance between the water absorption properties, such as absorption capacities under no load and under a load, particularly, absorption properties under a load, might be so low as to increase the amount of wet back in cases of absorbent articles having high resin concentration. In addition, there are also problems in that, generally, the post-treatment step for solving this blocking property is added to the surface-crosslinking step, therefore the production process becomes complicated.
Furthermore, there is a problem of the safety of the surface-crosslinking agent as used. Generally, in the case where the crosslinking agent is a low-molecular compound that has high reactive groups such as epoxy group, the water-absorbent resin as treated therewith has a relatively low blocking ratio under a load and therefore provides good results, but the crosslinking agent itself has property to stimulate skin. Thus, not only considering problems on environment of working, but also considering the application to sanitary materials, it is necessary to strictly control factors such as the amount of the crosslinking agent remaining in the resin, and further, complicated operations in the process are necessary also for decreasing the amount of the residual crosslinking agent. In addition, in the case where the crosslinking agent is a polyhydric alcohol, alkylene carbonate or the like, the crosslinking agent itself has relatively high safety, but some of them tend to increase the blocking ratio under a load, and further, have so low reactivity as the crosslinking agent that the reaction thereof needs a relatively high temperature and a long period of time. Therefore, during the crosslinking reaction, the water-absorbent resin might be degraded or the properties of the water-absorbent resin might be deteriorated.
Thus, in the actual state of things, there has never been an art (for obtaining water-absorbent resins as preferably used for sanitary materials) which is satisfactory with regard to the performance, the process, and the safety.